Prosecutors have asked a federal judge to force Boston mobster James "Whitey" Bulger to produce proof that he was awarded immunity by federal authorities before his June trial for his alleged role in 19 slayings spirals into an "unduly elongated and unmanageable" mess.

Prosecutors say the 83-year-old Boston gangster's FBI file makes no mention of the deal Bulger intends to testify he struck with former U.S. Attorney Jeremiah O'Sullivan in the 1970s. They say Bulger hasn't even presented proof that he and O'Sullivan ever met.

The Boston Herald reports the filing was made in response to the defense team's earlier demand that the judge compel the government to turn over "all correspondence" between prosecutors and investigative agencies.